It is well known to convert a hydrocarbon fuel such as propane to a synthesis gas containing hydrogen, carbon monoxide and nitrogen by oxidising the hydrocarbon with a sub-stoichiometric amount of air—such as by the use of a catalytic partial oxidation (CPOX) reformer. In small-scale applications this is preferentially carried out over a precious-metal catalyst to ensure the formation of gaseous products is favoured over the formation of carbon. The resulting synthesis gas may be used as a fuel for a solid-oxide fuel cell.
The carbon monoxide of the synthesis gas can either, through a reaction with steam be converted to carbon dioxide with hydrogen in the water gas shift reaction (WGS), or can be directly electrochemically oxidised to carbon dioxide at the fuel cell anode.
All fuel cells will operate on hydrogen and some fuel cell technologies, such as solid oxide fuel cells (SOFCs) can also operate on carbon monoxide or a hydrogen carbon monoxide mix. However, most fuel cells operate more effectively on hydrogen than they do on carbon monoxide.
Prior-art solid oxide fuel cells and fuel cell stacks typically operate in the range 700-1000° C. Under these conditions internal consumption of carbon monoxide though both the water gas shift reaction catalysed by the fuel cell anode, and direct electrochemical oxidation, are feasible. In these technologies, the temperature of internal reforming within the fuel cell anode occurs at temperatures close to that of the fuel cell, say around 800° C. and often the fuel cell anode is a thick film of over 100 microns thick.
In US 2002/0031695 at [193] there is disclosed a reference to the use of methanol reforming catalysts in the flow field of the anode plenum, to achieve internal reforming (i.e. in the fuel cell) and which relates to liquid/paste electrolyte fuel cells operating at low temperature. Though it is not stated, this must rely on the supply of water/steam for the substantial reforming to take place internally, with all the associated disadvantages, such as the need for a water supply, pump, control system, and their associated costs.
Prior art also indicates a preference for either internal or external reforming for fuel cell based systems using non-pure hydrogen fuels. External reforming occurs when the fuel is reformed to synthesis gas external to the stack. Internal reforming occurs when the fuel is reformed within the stack, often at or very close to the fuel cell anode. There are advantages and disadvantages with each method, with a common argument for internal reforming is that it is easier, results in a system with less parts and higher efficiency. Those practiced in the art know that full internal reforming requires considerable energy to be used in the reforming process, and that there is a chemical and thermal energy trade off to be made along with a control system requirement to enable the use of internal reforming to be effective. Internal reforming brings fuel cell material selection, integration and system control challenges for start-up, dynamic operation and shut down.
Other prior art includes U.S. Pat. No. 5,340,664, GB2405028, U.S. Pat. Nos. 4,374,184, 4,454,207 and US 2001/0010873.
Using an external CPOX reformer is not without its limitations. Not only does the resulting reformate stream become more dilute with the presence of nitrogen coming from the air supplied to the reformer, but there is always a risk of downstream reformer carbon deposition when the temperature drops below 700° C., e.g. within an intermediate-temperature fuel cell stack.
Likewise, the use of a WGS reactor, as its name suggests, requires the supply of water to the reaction site so that the carbon monoxide and water can be shifted to carbon dioxide and hydrogen.